(7he) Chemical Thermodynamics of Aqueous Atmospheric Aerosols: Modeling and Microfluidic Measurements

Lucy Nandy, Advisor: Cari Dutcher, University of Minnesota-Twin Cities

4th Year PhD Candidate

Research Interests:

Accurate predictions of gas-liquid-solid equilibrium phase partitioning of atmospheric aerosols by thermodynamic modeling and measurements is critical for determining particle composition and internal structure at conditions relevant to the atmosphere. Organic acids that originate from biomass burning, and direct biogenic emission make up a significant fraction of the organic mass in atmospheric aerosol particles. In addition, inorganic compounds like ammonium sulfate and sea salt also exist in atmospheric aerosols, that results in a mixture of single, double or triple charged ions, and non-dissociated and partially dissociated organic acids. Statistical mechanics based on a multilayer adsorption isotherm model can be applied to these complex aqueous environments for predictions of thermodynamic properties.

In this work, thermodynamic analytic predictive models are developed for multicomponent aqueous solutions (consisting of partially dissociating organic and inorganic acids, fully dissociating symmetric and asymmetric electrolytes, and neutral organic compounds) over the entire relative humidity range, that represent a significant advancement towards a fully predictive model. Moreover, the model has been applied to determine the activity of ions to predict pH of acidic aerosol particles. The model is also successful in predicting hygroscopicity of amino acids and other organic compounds in the atmosphere. In addition to the modeling approach, water loss of multicomponent aerosol particles is measured by microfluidic experiments to parameterize and validate the model. In the experimental microfluidic measurements, atmospheric aerosol droplet chemical mimics are generated in microfluidic channels and stored and imaged in passive traps until dehydration to study the influence of relative humidity and water loss on phase behavior.

These efforts together will enhance understanding of atmospheric aerosol phase, solid/liquid/gas partitioning, and liquid-liquid morphologies found in the troposphere. Additionally, although this research emphasizes atmospherically relevant systems, the measurements and modeling are useful to any application that requires thermodynamic predictions of water content, like in soil permeability, drying processes in food science, waste water treatment and industrial discharge.

Teaching Interests:

My teaching activities are focused on fluid mechanics and thermodynamics. I am currently designing a course on aerosol chemistry.

Course interests - Fluid Mechanics, Multiphase Flows (Macro- and Micro- scale), Thermodynamics, Heat Transfer

PhD Dissertation: “Thermodynamic Properties of Water Soluble Organics in Atmospheric Aerosols”

Under supervision of Prof. Cari S. Dutcher, Mechanical Engineering, University of Minnesota

Research Experience:

My research projects have been a combination of fluid mechanics, thermodynamics, aerosol chemistry and heat & mass transfer. My current project is thermodynamic modeling and measurements of atmospheric aerosol properties and phase behavior. The model is based on adsorption isotherm by statistical mechanics, and the measurements are done by biphasic microfluidic experiments. Collaboration with other research groups has aided me to find various applications of the model, like finding pH and hygroscopicity of aerosol particles. Prior to my doctoral studies, I have worked on projects in a steel industry that required a heat transfer model for dynamic secondary cooling control for uniform cooling of steel slabs, and a mixing model for alloy dissolution in gas-stirred steel ladle for complete homogenization. I have also worked on a small project during my masters’ on development of microchannel assisted cooling devices that involved image-analyzing interferometry. My masters’ dissertation project was numerical simulation of evaporative processes using ANSYS (FLUENT) that had applications in design process of two-phase cooling systems.

Teaching Experience:

In addition to my research career, my teaching role as a teaching assistant has helped me to be more analytical in my approach to learning and teaching. I have led discussion sessions for both fluid mechanics and thermodynamics for undergraduate students, and have also TAed applications of thermodynamics in fluid flows for undergraduate and graduate students. For all the TA positions, my duties also included holding office hours, grading assignments and exams, and very few guest lectures. Aside from that, I have mentored an undergraduate student in a mentor program in the college of science and engineering, and have mentored another undergraduate student in our laboratory who works on microfluidic droplet experiments. Additionally, I have taken a course over a semester in Preparing Future Faculty for teaching in higher education.

Future Direction:

Since I want to pursue my career in academia after completing the PhD program, I plan to continue working in the fields of aerosol science, chemical thermodynamics and multiphase microfluidics to gain new insights and future research responsibilities. I would also like to merge the knowledge with the topics that I worked on during my Masters that involved microscale heat transfer and computational fluid dynamics (CFD). I believe CFD will play an important role in multiphase microfluidics that uses water-in-oil droplets of volumes in the range picolitres to nanoliters.

I would like to take the current research further by developing thermodynamic models for complex liquid-liquid phase separated atmospheric aerosols to study the amount and composition of each phase and the degree of wetting. This will help predict properties of the liquid phases, and what factors are important to determine the equilibrium morphology of aerosols. Furthermore, liquid-liquid phase separation has applications in maintaining the physical stability of protein formulations. I would also like to explore this field that will enhance maintain stability across the protein therapeutics shelf life for desirable products.

Since microfluidic multiphase flows have a variety of applications like emulsions in food industry, mixing by mass transfer in a microreactor, micro separation, and particle production and synthesis, I would like to combine the heat transfer studies to understand microfluidic emulsifications for emulsion stability that can influence processes. This will address long-term storage stability of food-relevant compounds. A computational fluid dynamics (CFD) model can also be developed to study the flow field, heat and mass transfer to study internal flow patterns and dissolved chemical transfer across fluid interfaces. This can be extended to determine chemical reactions like nitration of toluene in the industries.

As a faculty, I will ensure that I collaborate with modelers and experimentalists for future research possibilities, and get realistic findings that can have various applications in aerosol and water studies. I will also impart my knowledge to students and help them see themselves having ability to understand science and engineering problems and how these problems exist in real concrete ways.

Publications:

Nandy, L. and Dutcher, C. S., Isotherm-Based Thermodynamic Model for Solute Activities of Asymmetric Electrolyte Aqueous Solutions; J. Phys. Chem. A (under review)

Craig, R. L., Nandy, L., Axson, J. L., Dutcher, C. S., Ault, A. P.; Spectroscopic Determination of Aerosol pH from Acid-Base Equilibria in Inorganic, Organic, and Mixed Systems; J. Phys. Chem. A (under review)

Marsh, A., Miles, R. E. H., Rovelli, G., Cowling, A. G., Nandy, L., Dutcher, C. S., and Reid, J. P.; Influence of Organic Compound Functionality on Aerosol Hygroscopicity: Dicarboxylic Acids, Alkyl-Substituents, Sugars and Amino Acids; Atmos. Chem. Phys., 17, 5583−5599, 2017. https://doi.org/10.5194/acp-2016-1051

Nandy, L., Ohm, P. B., and Dutcher, C. S.; Isotherm-Based Thermodynamic Models for Solute Activities of Organic Acids with Consideration of Partial Dissociation; J. Phys. Chem. A, 120 (24), 4147−4154, 2016. http://doi.org/10.1021/acs.jpca.6b01904

Rindelaub, J. D., Craig, R. L., Nandy, L., Bondy, A. L., Dutcher, C. S., Shepson, P. B., and Ault, A. P.; Direct Measurement of pH in Individual Particles via Raman Microspectroscopy and Variation in Acidity with Relative Humidity; J. Phys. Chem. A, 120 (6), 911-917, 2016. http://doi.org/10.1021/acs.jpca.5b12699

Nandy, L. and Dutcher, C. S., Phase behavior of ternary ammonium sulfate and dicarboxylic acids mixture solutions in aqueous aerosol mimics (in preparation)